This invention relates generally to a photoelectrical sensor and particularly to a photosensor with a filtered phototransducer amplifier circuit for reducing sensitivity to AC powered ambient light fluctuations of a predetermined frequency.
A problem which photoelectrical sensors of the type intended for use in environments including artificial light, such as remote control devices, security alarms and the like, is false detection due to artificial light fluctuations caused by AC powered ambient light sources. Such photoelectrical sensors are used in systems that employ light transmitters to generate pulses of light. These light pulses must be correctly sensed and decoded by appropriate detection logic circuitry of the photoelectrical sensor. If compensation for artificial AC light fluctuations produced at the frequency of full wave rectified standard AC line voltage is not provided, then such fluctuations alone, or in combination with other nonperiodic electrical noise, can cause the detection logic circuitry to falsely sense transmission of a valid light pulse from the light transmitter.
A number of patents address light noise problems. In U.S. Pat. No. 4,331,446 issued Apr. 16, 1983 to Fukuyama et al., mutual interference protection is provided. In U.S. Pat. No. 4,207,466 issued Jun. 10, 1980, to Drage et al., other light radiation and electrical signal noise sensed by the detector are filtered from the output signal according to the pulse width and frequency of pulse radiation generated by an emitter. A high pass filter is similarly used in U.S. Pat. No. 4,591,710 issued May 17, 1986 to Komadina et al. Other techniques are shown in U.S. Pat. Nos. 3,970,846 issued Jul. 20, 1976 to Shofield, Jr. et al. and 4,749,853 issued Jun. 7, 1988 to Salim.
A problem with such photoelectrical sensors is that in their amplification stage, insufficient filtering is provided for the level of amplification such that errors due to ambient light fluctuations can still occur. In addition, the pass bands response transition between attenuation to passing, or nonattenuation, is gradual which reduces the low frequency limit of the range of acceptable signal frequencies which must be sufficiently spaced on the frequency spectrum from the frequency of AC light fluctuation to achieve maximum insensitivity to ambient light. Since High pass transition can be located closer to the AC light fluctuation frequency, then the range of acceptable signal frequencies cannot be enlarged and the low pass transition cannot be moved downward for improved high frequency noise insensitivity.